FWB - I suspect part of it is that husbands take more pics of their wives, especially nudes, than vice-versa. I have to explicitly ask my wife to take pics of me, other than with the kids on vacations and such, and especially when I'm nude. She's just wired to think "mr. lovemonkey is nekkid, i need to take a pic" like I am with her.
And, relatedly, if we assume, I think correctly, that husbands set up and curate about 85%-90% profiles, then they might be biased against their own pics; they think all of their wife's pics are sexy, but don't care so much for their own so they feel they have fewer to post.
And of course as we know there are many couples who have some noticeable differential in their levels of physical attractiveness. In my unscientific experience this favors the females by a good margin, though definitely not in all cases. So, maybe they're just trying to put their best foot forward, and hubby's pics don't make the cut.

